After 13 days of searching by air and on the ground, Alaska State Troopers have suspended their search for a Wisconsin survival instructor missing in a remote area of the Northwest Arctic.

Thomas Seibold, 31, of Three Lakes, Wis., was first reported missing on Nov. 11 after he failed to make a planned contact ahead of a flight out of Kobuk.

The German-born traveler had been in Alaska since June, first spending time at a Southeast fish camp and then along the Tanana River near Fairbanks. In September, Seibold headed to the Northwest Arctic village of Ambler. He hiked 30 miles north to a cabin owned by a travel companion with plans, according to his colleagues from Wisconsin's Teaching Drum Outdoor School, to spend October exploring the area alone and then to hike 25 miles to Kobuk by Nov. 11.

What happened to Thomas Seibold next is so far a mystery.

"We believe things may have gone awry for him," said Sgt. Duane Stone, a supervisor for the trooper post in Kotzebue.

Seibold's last known communication came in the form of a letter or journal entry dated Oct. 7 and found in the cabin, Stone said. It mentioned that he was going out for a two-day trip.

"He wrote about his plans to hike out for further exploration and that is where the letters ended," wrote Lety Seibel, a colleague from the outdoor school, in a post on the school's Facebook page.

Other writings found at the cabin contained vague hints of possible plans, according to Seibel: a description of hiking and camping at higher elevations, notes on preparing wood at the cabin for upcoming weather.

"The cabin search also revealed that Thomas does not have a gun with him," she wrote.

Also left in the cabin, according to Stone: caribou hides and a heavy winter parka.

None of it has led searchers to Seibold, who friends describe as a thoughtful seeker on a quest to answer some of life's big questions in the Alaska wilderness.

In the past 13 days, troopers along with the Kotzebue Police Department, Northwest Arctic Borough Search and Rescue, local hunters and homesteaders and pilots have conducted aerial and ground searches over a 3,500-square-mile area of mountains, canyons and river drainages.

A few days in, a pilot found a circle marking on a gravel bar about eight miles north of the confluence of the Ambler River and Ulaneak Creek, according to Seibel. Colleagues think it may have been a sign denoting a place to drop provisions.

"Because the circle has such strong meaning to Thomas we think there's a high likelihood he etched the sign," she wrote in a Facebook post.

The circle is the only sign of a human traveler in the area, Stone said.

"We've flown low enough to see wolverine, wolf and bear tracks," he said. "We haven't found any sign of a human moving around at all in the area."

With little information to go on and conditions becoming hazardous for searchers, troopers decided to suspend their efforts after a final flight between the Ambler River and Shungnak River valleys on Nov. 24, Stone said.

Temperatures have been as low as minus 20 or minus 30 since October, dipping to minus 40 in recent days, he said.

Seibold has extensive experience with winter wilderness travel and survival, according to his Teaching Drum colleague Tamarack Song. He had completed a year-long "wilderness immersion" in northern Wisconsin and taken courses in the Arctic.

"If he's not injured and has adequate calories to keep himself going he's going to be all right," Song earlier told the Daily News.

But the area near the Ambler River that Seibold is thought to have been exploring is harsh even for Northwest Alaska, Stone said. Some local hunters and trappers avoid it because of overflow and difficult traveling conditions.

"It is remote and it is hazardous even on Alaska standards," he said. "If trappers aren't going up there it should give you kind of an idea."